INFLUENCE OF WATER ON PLANT-LIFE 25 
and if this conditionJis prolonged the plant dries up 
and dies. 
.4, The materials derived from the soil and absorbed 
by the roots are conveyed in a current of water which 
passes up the stem to the leaves. This ascending stream 
of sap is called the transpiration-current, and the main- 
tenance of its flow is necessary to the healthy life of the 
plant, for, like the stream of blood in animal bodies, it 
conveys a cargo of materials by the utilization of which 
the plant is enabled to live. The 
path of the water is through the 
woody tissues which constitute the 
greater part of the vascular system 
Fic. 2.—LoNGITUDINAL SECTION OF PART OF A 
Roor-Trr, SHOWING OUTER TissuES OF Root 
(a) AND Roor-Harrs (6), SURROUNDED BY 
Sort-ParticuEs (c). (HicHiuy MAGNIFIED.) 
Fic. 1.—Srepurnc or inroots and stems. From the stems 
“eee een great trunk-veins are given. off to 
Diarcnan Sum) |. bhe leaves, where they break up 
into a network of capillaries. Here 
the water is yielded up to the living cells, and with it 
the nutrient material contained in it. 
Root-Absorption.—Most of the higher plants live rooted 
in the ground. Water enters the plant by the roots. 
The actual entrance is effected through the root-hairs— 
tiny, hair-like cells which are found clothing the roots 
near their tips (Figs. 1 and 2). The water passes through 
the walls of these root-hairs by a physical process known 
as liquid-diffusion, or osmosis (see p. 91). 
